clunk on startup

Toro recycler, Model 20377, 4yr old, used sparingly, just happened, pull starter rope, sounds like metal clunk at startup and once at stop, but starts and runs fine. It sounded like the blade hit a metal bar on the ground. I hit nothing on the ground. Nothing there, nothing loose that I can find. It is reproduceable every time I start it and this just began. Where do I look?

I've done all of that and found nothing. A few strands of grass were lodged in the perimeter of the housing, so minor that I didn't notice it on first inspection. I removed it so it is now pristine under there. Blade bolt is in excess of 60 foot pounds, not loose, no wobble, blade smooth. Can't imagine strands of grass creating a metal clunk, but noise is better at startup and gone when stopping. Removed pull starter and found nothing loose or foreign. I am stumped!